The scraped-surface heat exchanger (SSHE) is a well-established device used in continuous-flow heating or chilling of foodstuffs, pharmaceuticals, lubricants, and other materials requiring such thermal alteration (i.e., “products”). An additional effect of the use of an SSHE is stirring of products, which can involve combining ingredients, whipping gases into liquid materials, and other alterations to products. As currently manufactured, a representative SSHE can have a heat exchanger tube some 6″ in diameter and roughly ten feet in length and can have a motor that provides on the order of four horsepower. SSHEs can be made that can operate in a horizontal or vertical position. Some are designed to be shifted to vertical orientation before disassembly for inspection and scraper blade exchange. The SSHE has met with excellent market acceptance, as a serviceable tool for modern industry.
An exemplary SSHE consists of a set of concentric layers, where the outermost layer is metal-skinned thermal insulation for safety, energy economy, thermal uniformity, and process control. Inside the insulation is the outermost pressure tube. Next inward is a chamber into which the thermal control medium—e.g., live steam, ethylene glycol, ammonia, fluorocarbons, or another heat exchange fluid—is introduced. The next inward layer is a second chamber, through which the product to be heated or cooled is forced. The innermost layer is a drive shaft that carries the scraper blades.
Continuous pressure keeps the product flowing longitudinally through the SSHE and prevents uncontrolled air from getting into the SSHE. Relative rotation between the drive shaft and the rest of the structure causes the scraper blades to scrape the product away from the thermally controlled product-chamber surface (i.e., the “scraped surface”). Scraping a layer of product away from the surface in a completely filled environment causes another portion of product to contact the scraped surface. The combination of forced flow and scraping determines the amount and duration of direct exposure of product to the scraped surface so that the cooling or heating meets process needs.
The scraper blades are free to pivot out from their connection to the drive shaft towards the thermally controlled scraped surface, until the leading edge of the blades contacts the scraped surface against which the blades are held by resistance from the fluid flow properties of the product when the drive shaft is rotated. Multiple blades staggered at different radial and axial orientations along the drive shaft provide enough overlap to ensure that the entire inner surface of the heat transfer tube is scraped continuously. A drive motor and appropriate mechanism provide torque for the relative rotation of the SSHE parts. Suitable seals prevent leakage and contamination.
Typically, the scraped surface must meet some or all of the following requirements. It must be chemically compatible with both the product being processed and the thermal control medium. It typically also must be suitable for contact with food or pharmaceuticals if the product is intended for human use. It must be reasonably efficient in transferring heat to and from the thermal control medium and the product involved. It is also generally desirable for the scraped surface to be significantly harder than the scraper blade, since the scraped surface is much more expensive and difficult to service and/or replace than the blades.
The scraper blades also typically must meet certain requirements. Various styles and materials for scraper blades have been used since the introduction of the SSHE. For example, blades have been made from stainless steel and other metals by machining them from flat stock. Such blades can show a wear pattern that leaves a feathered edge of blade material projecting outward along the scraping surface. Maintenance of blades to remove feathering requires disassembling the SSHE and grinding the blades to the original profile. After several grindings, the blades become too short to maintain a working angle of attack against the scraped surface that is within the acceptable range, so that the blades must be discarded. An additional consideration for metal scraper blades is the requirement that the heat transfer tube surface be extremely hard, which requires special production steps and periodic inspection, adding to production and maintenance costs.
Some plastics have proven suitable for blades, but are limited by the temperatures and friction environment of the SSHE. One, polytetrafluoroethylene (PTFE, commonly known by the Dupont® tradename of Teflon®) is FDA and/or USDA recognized for use on foods and other critical materials. However, PTFE cannot be molded, but must be machined, making it useful for prototypes but in some instances less desirable for mass production. Other plastics, such as Polyoxymethylene (Acetal) and Polyphenylene sulfide (PPS) can be significantly more long-wearing than PTFE in some applications and are injection moldable, which lowers cost if demand is sufficient. Most plastics considered for SSHE applications wear away without leaving a feather edge like metal, so such blades do not require sharpening. Being softer than stainless steel or nickel, they obviate the need for a hard inner surface coating on the SSHE heat transfer tube. Fillers—finely divided fiberglass, carbon filaments, and even talc—can improve some plastics' mechanical properties. A few fillers are FDA/USDA approved.
A disadvantage of the plastic blades described above is that even the most suitable of the plastics often exhibit more rapid wear than metal blades. This can obligate users to halt process operations more often than periodic maintenance requires, just to inspect or replace blades. Such halts, adding extra cooldown, cleanup, disassembly, and other maintenance steps, can add cost and interfere with production. Further, many plastics are not recognized by the U.S. Food and Drug Administration (FDA) or the United States Department of Agriculture (USDA) for use on foods, either in virgin form or with fillers.
Accordingly, it is desirable to provide a scraped-surface heat exchanger scraper blade that combines desirable manufacturability and scraping performance and improved blade life while having desirable SSHE properties.